Doctrine is at the center of law and legal analysis. This Article argues that we have fundamentally misunderstood its nature. The conventional approach to legal doctrine focuses on theory and applications. What is the doctrine designed to do and how does it function? But many doctrines cannot be adequately understood or evaluated under the conventional model because they contain an additional, hidden element. They are built on foundational facts: potentially contested factual assumptions embedded in the doctrinal structure itself. Foundational facts are judges' generalized and invisible intuitions about how the world works. Whether a defendant acted in a particular way out of a particular motive are decisional, rather than foundational, facts. But the likelihood of actors in defendant's position acting that way or having that motive are foundational facts, and doctrinal rules - including burdens of proof and standards of review - will be structured differently depending on whether judges assume a high or low likelihood. Foundational facts thus drive doctrine. Without an understanding of a doctrine's foundational facts, we cannot adequately understand the doctrine and its changes over time. Foundational facts only come to light when doctrine shifts, seemingly inexplicably and often without judicial acknowledgment that anything has changed. That doctrinal shift serves as a cue to look for changed foundational assumptions that might be driving the doctrinal change. Identifying those foundational facts, in turn, allows us to better understand and evaluate both the doctrine and its underlying assumptions.